JSON Web Token (JWT) — Structure Validation Regex for JavaScript

What this pattern does

This page provides a well-structured, multi-part regular expression for matching json web token (jwt) — structure validation, ported and verified for JavaScript. In security-sensitive code, using an unverified regex can open the door to both false positives and denial-of-service attacks. The snippet below is ready to drop into your JavaScript project — whether you're validating in an Express middleware, a Next.js API route, or a client-side form.

Javascript Implementation

// JSON Web Token (JWT) — Structure Validation
// ReDoS-safe | RegexVault — Security > API Keys & Tokens

const jsonWebTokenJwtStructureValidationRegex = /^([A-Za-z0-9_-]+)\.([A-Za-z0-9_-]+)\.([A-Za-z0-9_-]+)$/;

function validateJsonWebTokenJwtStructureValidation(input: string): boolean {
  return jsonWebTokenJwtStructureValidationRegex.test(input);
}

// Example
console.log(validateJsonWebTokenJwtStructureValidation("eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIn0.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c")); // true

Test Cases

When to use this pattern

This pattern is drawn from the Security > API Keys & Tokens category and carries a ReDoS-safe certification. That matters for JavaScript developers because especially critical in long-running Node.js event loops where a ReDoS vulnerability can block the entire process. RegexVault audits patterns against known backtracking attack vectors, ensuring you have the necessary context before using this regex in a high-stakes production environment.

Common Pitfalls

Technical Notes

Structure only — does not validate header algorithm, payload claims, expiry, or signature. Three segments: header (base64url) + payload (base64url) + signature (base64url, may be empty for 'none' algorithm — always reject 'none'). Capture groups: 1=header, 2=payload, 3=signature.